Excimer laser annealing (ELA) is an established process for preparation of thin-film transistor (TFT) back-planes in the production of high quality display panels. Typically ELA involves irradiating a panel coated with amorphous silicon with a line of radiation projected from an excimer laser-beam. The excimer laser radiation changes the amorphous silicon to polycrystalline silicon.
Electrical performance of the TFTs takes advantage of a modified microstructure that is generated when the amorphous silicon is changed to polycrystalline silicon. The arrangement of crystallites of the polycrystalline silicon can be important for a specific TFT circuit design. Usually the crystallite arrangement follows a high degree of order after iterative excimer laser irradiation. It has been found that angular distribution and polarization of the laser radiation can affect the crystallite arrangement.
Excimer-laser line-projection apparatus is extensively described in the prior-art. By way of example, such apparatus is described in detail in U.S. Pat. No. 7,428,039, and in U.S. Pat. No. 8,937,770. Optical combination of beams for increasing irradiation power in such apparatus is described in detail in U.S. Pat. No. 7,408,714. Synchronization of pulsed beams from beam-combined lasers is described in detail in U.S. Pat. No. 8,238,400. All of the aforementioned patents are assigned to the assignee of the present invention, and all are hereby incorporated herein by reference.
An effective means of controlling polarization distribution in excimer-laser line-projection apparatus is not described in the prior-art. In general, excimer lasers used in such apparatus do not emit a plane-polarized beam, and causing such a laser to emit a plane-polarized beam would result in a substantial (50%) loss of available power. In actual laser projection apparatus, some degree of polarization (ellipticity) may be introduced into a beam by non-normal incidence reflections from turning-mirrors, attenuators, or passage through birefringent optical components. This will be incidental, and will certainly not provide for a controlled angular distribution of polarization at any incidence point in a projected line of radiation.
There is a need for excimer-laser line-projection apparatus that can provide a controlled distribution of polarization at any incidence point in a projected line of radiation on a silicon-coated panel. Preferably, this controlled distribution of polarization should be achieved without substantial loss of laser power, and should maintain the angular distribution of the incident radiation.